Chapter 8 

Milling Operations 


Stupidity is the basic building block of the universe. 

 Frank Zappa 

Introduction 

Unlike lathes, which have been known for thousands of years, milling 
machines are less than two hundred years old. Because they require much 
more power than hand-driven lathes, their introduction had to wait for the 
invention of industrial water and steam power. Also, all their mechanical 
components had to first be made available, such as accurately fitted slides, 
large castings to resist cutting forces, calibrated leadscrews, and hardened 
steel cutting tools. 

Eli Whitney is credited with inventing the first milling machine about 1818, 
but the knee-and-column support arrangement of the universal milling 
machine of Joseph A. Brown (later of Brown and Sharpe) dates from 1862 
and marks an important step in the machines development. During the last 
half of the nineteenth Century, milling machines gradually replaced shapers 
and planers which have lathe-type, single-point tool bits that move over the 
work in a straight line and scrape off metal one stroke at a time. Milling 
machines, with their continuous cutting action, not only remove metal faster 
than shapers and planers, they perform additional operations like cutting 
helices for gears and twist drills. Today, milling machines greatly outnumber 
shaping and planing machines. Americans in New England and later the mid-
west continuously added features leading to the modern milling machine. 

Another important development came in the 1930s when Rudolph Bannow 
and Magnus Wahlstrom brought out the Bridgeport-style vertical milling 
machine. This design offers versatility and economy in place of the higher 
metal removal rates of traditional horizontal milling machines. Because of this 
versatility, there are more Bridgeport-style mills in existence today than any 
other milling machine design. Horizontal mills are now usually reserved for 
production applications where high metal removal rates on identical parts are 
needed, not prototyping and short runs. Bridgeport-style machines are also 


CHAPTER 8 MILLING OPERATIONS 

called knee-and-column machines and turret mills. The key features of these 
machines are a: 

 
Knee-and-column support for the milling table, which provides vertical 
motion of the work with respect to the tool. 
 
Saddle which supports the table to provide in-and-out motion from the 
vertical column. 
 
One-piece tool head which holds the motor, drive pulleys, and spindle. 
 
Sliding overarms or rams were eventually added to allow the tool head to 
be moved in or out with respect to the vertical column. Some machines 
have provisions for the tool head to be tilted side to side or back to front. 
The Bridgeport-style machine offers many advantages over the older 
horizontal milling machine design: 

 
The biggest advantage is the quills ability to advance and retract the 
cutter easily without cranking to raise and lower the milling table. This 
speeds production and reduces operator fatigue. The retractable quill lets 
the operator quickly withdraw the tool to clear chips from a hole or check 
its progress. Tactile feedback through the quill feed handle or handwheel 
also tells the machinist how the tool is cutting and lets him optimize feed 
with less danger of tool breakage. Vertical table movement is still 
available for high-accuracy depth adjustment or when more force on the 
tool is required. 
 
The second largest advantage is the Bridgeport-style machines ability to 
make angle cuts. With the horizontal milling machine, either the milling 
cutter is made on an angle, or the work must be positioned at an angle to 
the spindle axis. With the Bridgeport-style machine the operator merely 
needs to tilt the spindle to make an angle cut. Of course, the Bridgeport 
can also use an angled cutter or mount the work on an angle. 
 
Vertical milling machines must use smaller cutting tools than horizontal 
mills because they have less rigid, less massive castings, and lower 
horsepower motors. Still, they can accomplish the same end results as the 
horizontal mill, just more slowly. 
 
Vertical milling machines are less complex than horizontal machines 
because the one-piece tool head eliminates the need for complicated 
gearing inside the vertical column. 
 
Bridgeport-type milling machines usually have 1 to 5-horsepower motors, 
and smaller castings than most horizontal mills. Because of this they 
generally cost less. 
 
Knee-and-column mills offer versatility and economy in place of the high 
metal removal rates of traditional horizontal milling machines. 

MACHINE SHOP ESSENTIALS 

There are between 15 and 36 milling machine designs or styles, depending on 
who is counting, but the focus of this study is the Bridgeport-style vertical 
knee mill because they are most often used in shops doing prototyping and 
R&D work. They outnumber all other designs combined. This design has so 
much to offer that it has been copied in every industrialized country. At one 
time there were no less than thirteen separate Spanish companies building 
Bridgeport-style mills. A working knowledge of a Bridgeport-style vertical 
milling machine also provides a good start for operating any other style 
milling machine. 

Lathes and mills are complementary machines. While lathes rotate the 
workpiece and produce a cylindrical cut, milling machines move work into a 
rotating cutter and make a straight line cut. Lathes and mills are both capable 
of boring large-diameter holes, but mills are better at placing holes anywhere 
on the surface of the work. Although one can sometimes make do with just a 
lathe or mill, a well-equipped shop must have both machines. 

The lathe cutting tool is in continuous contact with the work and so makes a 
continuous cut. Milling machines are just the opposite. They use multi-tooth 
cutting tools and their cutting action is intermittent as each tooth takes a bite. 
Metal is removed in small individual chips. Unlike lathe cutting tools, end 
mills, the most common cutting tool for Bridgeport-style mills, cannot be 
sharpened freehand because they must be perfectly symmetrical. Sharpening 
them requires special fixtures and shaped grinding wheels. Smaller shops send 
their cutters out for sharpening. 

Adding a digital readout (DRO) is a great convenience to any milling 
machine. It reduces the need to repeatedly stop the mill to make 
measurements and lowers the chance of errors. When reset to zero, the DRO 
displays the exact displacement from a reference point on the workpiece 
making it possible for the operator to work directly with the dimensions on his 
working drawing. 

For production applications, there are large, expensive milling machines with 
three or more axes under computer control. Some machines perform all 
operations including automatic tool changing. However, today there is an 
intermediate step between a manual mill and a fully automated one. Adding a 
computer, digital readouts, and actuators to the X- and Y-axes of a 
Bridgeport-style mill does this. Not only can this enhanced machine tirelessly 
perform all its existing repetitive functions, it also has added new capabilities. 
Now the mill can engrave (drive the tool to cut numbers and letters in various 
sizes and fonts), cut radii and angles without a rotary table, make islands, 
pockets, and cut ellipses, and frames. Entering the position, diameter and 
number of holes, automates cutting a bolt-hole pattern; the system does the 


CHAPTER 8 MILLING OPERATIONS 

math. The computer can automatically compensate for the reduced diameter 
of resharpened milling cutters, saving time and money. The system can be 
manually programmed through its control panel, use stored programs, learn 
new tasks by memorizing a series of manual operations as the operator makes 
the first part, or accept files from CAD programs. 

In this chapter we will study two mills, the Sherline miniature milling 
machine and the classic, full-sized Bridgeport by Hardinge. First, we will 
look at each machine, then examine its major components and study each of 
its adjustments and controls. Next, we will look at milling machine cutting 
tools and accessories, and learn its step-by-step operation. Finally, we will 
review milling machine safety issues. 


